Hi, All,
I am building a amp with 50V-0-50V AC recommendation.
I have (2) 500VA, 45V tap transformer, and I have bunch of 80V capacitor laying around.
Is it advisable to use the 80V capacitors, or should I buy 63V capacitors?

Hi,
your 45-0-45 transformer will give about +-67Vdc after the rectifier and caps at standard mains voltage and about +-71Vdc when mains is running @ +6%.
Do not use anything less than 75v for smoothing caps and decoupling. Some designers who want very good long term reliability and low warranty claims might even specify 100V here.
The 50-0-50 transformer is probably aiming for about 200w / 8R power output but you will have about 160w / 8R (-0.9db).
To improve your output slightly you can add 2 low current (500mA) windings onto the toroid and rectify these. About 5Vac will give about 7Vdc. add these to the + & - rails to give +-74Vdc off load and use these higher volts to power the input and driver section. If you want you can go even higher and then regulate the driver rails down again (might improve sound quality).
hope this helps.

Hi again
forgot to add, the Vrail voltage will fall to about +-62Vdc when on full load,i.e. when driving about 7R.
Your 500va will drive 8R all day no problem and give good short term power into 4R when on music type signal but will run a bit warm if you drive 4R really hard e.g. disco, PA, sine wave drive to industrial etc.

Although you can get away with a 63v cap, it is NOT advised. If you have fluctuations on the mains, you will blow your caps pretty fast. I'd say 75 vdc is ok or even 80 vdc depending on what is available.

Also some Toroids actually measure MORE at idle so you may end up with your toroid actually being 47-0-47 in which case your unloaded rails are a tad over 65vdc after rectification losses.

With this supply and a 500 VA Toroid, you are looking at approx 180 watts 8 ohms with dynamic headroon of 230 watts or so. The better the PSU regulation, the higher your output.

Hi,
I hope you have the info you need, I have reviewed the various posts and ALL seem to be correct or very close. But you will need to read each response in context.
I do not know if one of our contributors was trying to wind me up but this is all the comment I will make on this thread.

Let's start at the beginning. Your project amplifier recommends a 50V-0-50v AC secondary transformer. No power rating was given, right?

Transformer manufacturers rate their transformers AT FULL RATED LOAD (any that don't, should, to conform) in other words if it's a 300W (VA) transformer it will be at a draw of 3A! Now, that will only occur when the amplifier, assuming Class AB not Class A, is flat out! At idle the transformer will be some X% higher than the 50V per side. This is called the regulation and usually around 6%for 300VA toroids, and around 4% for 500VA toroids.

So for a 300VA with 6% regulation the idle supplies will be about 50x1.414x1.06- 0.6 = 74V assuming nominal mains and negligible ripple at idle.

Ideally, you should use 80V caps to allow for higher than nominal mains noting that a further surge voltage (short term) allowance is built in for extreme surges.

Having said that I'd suggest you don't consider the 500W 45V tapped tranny, unless it's 45-0-45. Assuming it is and it has 4% regulation then your supplies would be 65V and you could use 75V caps or if desperately penny pinching use 63V caps with 3 power diodes in the feed. But they would be on their limit with quality of manufacture their only saving grace.

Considerable amplifier power would be lost as a result of perhaps 8V of loaded supply. A 150W amp -> a 100W amp! If the topology of the amplifier proper is designed with very high PSRR then the lower VA tranny and higher supply will allow a sizeable dynamic headroom and you could have effectively a 250W amplifier in a 150W guise.

A 150W/ch amplifier as above can be designed using a 300VA toroid! Utilizing the crest factor of typical full range musical programme, it would consume probably 80W while audibly clipping at the peak equivalent of 250W/ch.